IEEE Electrification - June 2021 - 14

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Figure 4. A bidirectional buck-boost dc-DC converter with (a) a cascaded capacitor and (b) a cascaded inductor in the middle.
DAB converter to develop a resonant DAB converter
[Figure 6(b)] to realize the ZVS turn-on within a wide
range of voltage and power. LLC resonant converters
are widely adopted as the dc-dc conversion stage of EV
chargers with high-power density and high efficiency.
Figure 6(c) shows the full-bridge LLC resonant converter
with a full-bridge diode rectifier serving as the secondary
side circuit. As shown in Figure 6(d), the LLC
converter can be further developed to be a bidirectional
converter called a CLLC by replacing the diode bridge
with one full-bridge circuit and adding one resonant
capacitor at the secondary side. The DAB and LLC converters
can easily achieve ZVS with a minimal switching
loss on semiconductor devices and limited EMI
noise emission. Therefore, they are recommended for
high-power applications.
There are also various types of isolated current-fed
topologies, including full-bridge, L-type, push-pull, and
L-type half-bridge circuits. Figure 7 shows some typical
single-phase current-fed converter topologies. Owing to
boost ability, the isolated current-fed converters are suitable
for FCV applications with a high step-up voltage
gain. Moreover, the input current ripple and turn ratio of
the high-frequency transformer in current-fed converters
are lower than those in voltage-fed dc-dc converters,
making them more desirable for FCV applications.
Because the current-fed converters have a shootthrough
immunity and low current stress, they are suitable
for low-voltage, high-current applications. In
isolated current-fed dc-dc converters, a controllable
clamping circuit is usually used to eliminate the voltage
spikes on devices. The current ripple of the L-type fullbridge
and push-pull topologies is low due to the interleaved
operation. Unlike other single-phase current-fed
topologies, where a two-winding transformer is used,
the single-phase push-pull converter [Figure 7(c)] uses a
three-winding transformer.
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Figure 5. A modular switched-capacitor dc-dc converter.
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IEEE Electrification Magazine / JUNE 2021
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Interleaved Topologies
For high-power applications, dc-dc converters may suffer
from high voltage and current ripples, large volume,
and heavy weight. To address these challenges, interleaved
topologies can be used. Figure 8 shows the schematic
of an eight-phase interleaved boost converter.
Each phase can be designed and implemented in a
modular way. The gate drive signal for each phase is
shifted by 2π/N (N is the total
number of phases; here N = 8). All
phase currents would have the
same waveform, except that they
are shifted by an angle of 2π/N.
The overall current ripple in the
input side, which is the sum of all
of the input phase currents, is significantly
reduced because of the
ripple cancellation effect, shown
in Figure 9. It should be noted that
the frequency of the input current
ripple is increased to N*fsw, where
fsw is the switching frequency.
Because of the lower current ripple
and smaller harmonics, the
size of the filter on the input side
can be greatly reduced. There is
an output filter capacitor for each
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IEEE Electrification - June 2021

Table of Contents for the Digital Edition of IEEE Electrification - June 2021